The present invention relates to method for increasing the data rate without impairing the signal quality in a communications system for signaling over electric distribution lines from the customers' premises to the distribution substation. This inbound signaling concept has been described in detail in United Kingdom Pat. No. 1,575,026 complete specification published Sept. 17, 1980, the disclosure of which is hereby incorporated by reference.
The basic inbound signaling pattern described in said United Kingdom Pat. No. 1,575,026 occupies two cycles of 60 Hz. FIGS. 1A, 1B, 1C, 2A, 2B, and 3 are reproduced from the corresponding figures in said United Kingdom patent. In order to create a signal pattern for FIG. 1A or 1B a pulse of current is drawn at the customer's meter (on the utility side of the meter) for a very brief period just before the voltage zero-crossing in one of these two cycles; this current does not occur at the same time in the adjacent cycle. Current must flow from the substation to supply this current pulse on the customer's premises; thus the signal can be sensed by examining the current pulse at the substation as shown in FIG. 3. The current pattern produced is very different from that of normal loads that tend to remain the same in successive cycles. Detection of the signal at the substation is performed by subtracting the current in one cycle from the current in the next cycle. If only normal loads are present these currents will be the same in the two cycles and the difference will be zero. In the case of a signal the current pulse is present in one cycle and not in the other. Subtraction leads to either a positive or a negative difference between the two cycles.
To guard against mistaking a change of loads for a genuine signal, the signal pattern is repeated four times to transmit each bit of information reliably. Two current pulses are actually used in the basic signaling pattern. The locations of these pulses within the two cycles of the signaling pattern are shown shaded in FIG. 1C.
An experimental facility designed along the lines described in the referenced U.K. patent was created at the West Methuen substation of Massachusetts Electric Company. Testing revealed that the signal strength attained in this facility was in excess of that required to achieve high quality signaling; it was judged that reliable communication could be achieved using a signal of half the strength used in the test at West Methuen.
By halving the signal strength one could expect to double the data rate. This improved data rate could be realized either by shortening the bit pattern to four cycles wherein the basic pattern of FIG. IC was repeated only twice, or by maintaining the eight-cycle bit but employing therein only the signaling pattern of FIG. 1A or 1B. In this latter option the doubling of the data would be achieved through the use of two simultaneous transmissions. One should note that this approach which employs a single type of signal in each pattern can be implemented using a transmitter with only one silicon controlled rectifier for switching the signaling resistor--by contrast the four-cycle bit approach would require two SCR's to produce the two types of signal pulses.
Emerson Electric Company of St Louis Mo. has been granted a license to build and sell communications equipment based on the technologies pioneered by New England Power Service Co. and described in said United Kingdom Pat. No. 1,575,026 and in U.S. Pat. Nos. 4,106,007 and 4,105,897. An account of the equipment was given recently in a paper entitled "TWACS--the 60 Hz Power-Line System" presented to the Pacific Coast Electric Association on Mar. 13, 1980 by Julius Orban.
The Orban paper describes the technology based upon a bit of duration corresponding to eight periods of the 60 Hz waveform--this is the duration of the bit used as West Methuen test site. The paper reports that more than one inbound message can be transmitted simultaneously (such as might be achieved using the patterns of FIGS. 1A and 1B for two types of bits--as explained above). The paper reports that "up to four" simultaneous messages can be used in the "inbound" (load-to-source) direction presumably by employing the patterns of FIGS. 1A, 1B, 2A and 2B for the four types of messages.
The technology described herein permits signal strength equal to twice that achievable through the use of any one of the signaling patterns of FIGS. 1A, 1B, 2A and 2B in the manner suggested by the Orban paper. In addition it provides up to fourteen simultaneous binary messages with the preferred configuration being up to seven simultaneous messages in quaternary code, using eight-cycle characters.
One may understand the essential feature of the present invention by references to the aforementioned patents and the power-line communication systems described therein. However, one should recognize that the invention is capable of wider application than the specific application detailed above. In fact, it is applicable to any signaling scheme wherein each signaling event is capable, at least in principle, of representing a bit of intelligence. As used herein the term "signaling event" denotes the single application and the subsequent removal of a signaling load. One finds, for example that this invention can be applied to increase signal quality and to simultaneously improve data rate in the "outbound" signaling scheme which is marketed by General Electric Company, Ltd. of England under the name "Cyclotrol". On the other hand, this invention is not applicable to the signaling scheme of Haberly, as described in U.S. Pat. No. 3,509,537, since in that scheme one requires a plurality of signaling events to represent a single bit of intelligence.
In the present invention the signaling patterns can be organized in such a manner that a four-fold increase in signal strength can be realized by lengthening the bit to eight cycles rather than the basic two cycles, and that by lengthening the duration of each pattern one can actually realize an increase rather than a decrease in data rate. This seeming contradiction that the data rate can be increased by employing patterns of longer duration arises from the use of orthogonal patterns of signaling events over intervals that persist for an even multiple of the basic signaling event.
At this point, some attention should be given to the properties of these patterns which have been described previously as "orthogonal". These patterns occur in pairs; the cycles which contain signaling events for one member of the pair do not contain signaling events for the other member, and vice-versa. The signal that is sensed when a member of a pair is detected will be the maximum signal that can be accumulated from the signaling events therein--which implies that all of the samples from cycles containing signaling events are accumulated in the same sense. The signal sensed from the two members of the pair will be equal in magnitude but opposite in sign.
The orthogonal feature is that the detection mode appropriate to the sensing of the members of one pair of patterns will accumulate no net signal if it is applied to any pattern from another pair of patterns. This property must prevail for all detection modes and the associated pairs of patterns--thus, clearly, there will be a limited number of such pattern pairs that can be associated with patterns of a given duration. The totality of the pairs of patterns and their properly associated detection modes which exhibit this orthogonal property can conveniently be referred to as an orthogonal family.
It is contrary to one's intuition that an increased data rate is to be realized through the use of patterns which prolong the duration of individual bits--it is obvious that one can attain an enhanced signal quality through the use of longer patterns, but one would normally expect this to be associated with a reduction in data rate as was the case in the teaching of the above-mentioned United Kingdom patent.
The present invention relates to the manner in which one can attain this simultaneous enhancement of signal quality and of data rate.
The principal object of the invention is the realization of improved data rate while improving signal quality in communication technologies wherein each signaling event is capable, at least in principle, of representing a bit of digital intelligence.
An object of the invention is the realization of simultaneous, synchrous transmissions of multiple noninterfering messages over the same communication medium.
It is a feature of the present invention that number bases greater than two are used in the transmitted messages to produce messages of shorter duration.
It is another object of the invention to achieve an enhanced data rate in transmissions where it is not possible to provide the synchronization needed to permit transmissions from a multiplicity of transmitting sites.